Allow mentors to assign submissions to themselves

Now when users view a submission they can assess, they see this:

![screen shot 2013-06-17 at 4 31 47 pm](https://f.cloud.github.com/assets/124687/664774/39b753fe-d78d-11e2-94c8-54d51ce021fc.png)

When they click the "Begin Assessment" button, they see this:

![screen shot 2013-06-17 at 4 32 18 pm](https://f.cloud.github.com/assets/124687/664783/52192b0c-d78d-11e2-9d65-10e8153e4822.png)

For the next 15 minutes, anyone else who has permission to view the submission sees this:

![screen shot 2013-06-17 at 4 32 02 pm](https://f.cloud.github.com/assets/124687/664791/6f246112-d78d-11e2-970a-d8a2bf97207e.png)

Once the 15 minute window expires, anyone who views the submission will see the "Begin Assessment" button again. The original assignee may also click the "Cancel Assessment" button on the review form to release themselves from the assignment early, if they wish.

This fixes #69.